The present disclosure relates to apparatus and methods for tracking the location of assets in wireless or partially wireless communication networks. In particular, the disclosure relates to identifying asset location using the time-of-arrival (hereinafter, TOA) at a receiver of a communication sequence or sequences broadcast by a movable transmitter. TOA estimation may be referred to as “time-stamping” of a communication sequence.
Precise estimation of communication sequence arrival times may be desirable when mobile asset location is determined using ranging or triangulation techniques in connection with receivers present in a communication network. As the number of assets, the amount of communication traffic, or the rate of time-stamping events in a network increases, time-stamping performance and overall network performance may decrease. Usually, it is not desirable to identify the location of every wireless asset present in a network in connection with every broadcast packet. Networks often operate inefficiently, however, because without time-stamping every received packet, it may be impossible to time-stamp the desired packets.
In some wireless networks, access point architecture may limit network performance. Often, access points do not permit time-stamping processes to be performed quickly enough to generate accurate estimates of wireless asset location.
Wireless communication networks often have a cellular architecture in which adjacent cells operate on different channels. To minimize interference between communication signals, the cells are arranged to maximize the physical distance between the channels. Although this arrangement maximizes the spatial bandwidth available to network users, it often degrades location estimation accuracy by decreasing the density of receivers available for time-stamping on a given channel.
Wireless communication networks are increasingly designed to use 802.11 radio frequency signal structures. As this standard proliferates it will become increasingly valuable to identify the location of wireless assets broadcasting standard 802.11 packets.
Often, it is desirable to identify the location of “passive” assets present in the vicinity of a wireless communication network. For example, it may be desirable to track the location of pallets in a cargo yard. One solution is to attach active tags to the passive assets. Often, tags transmit specialized signals on a fixed schedule and on a single frequency. When a network does not receive a scheduled transmission, location information may be lost. Fixed schedule tags can not “choose” when to transmit and so can not optimally utilize periods of open “air time.” Location estimation may therefore be particularly difficult or inefficient in high traffic communication networks. Single frequency tags may not be optimally trackable in multiple frequency cellular networks.
Time-stamping schemes often use correlation-based signal processing techniques (similar to signal decoding techniques). Noise inherent in known correlation techniques (e.g., cross-correlation artifacts) can degrade time-stamping accuracy.
Multiple signal arrivals (hereinafter, “multipath”) from a single communication sequence transmission may degrade decoded signal quality and make sequence detection difficult. Multipath may occur when structures near a transmitter produce transmission echoes that arrive at the receiver after the “line-of-sight” signal. The “line-of-sight” (hereinafter, “LOS”) signal is the portion of a transmitted signal that traverses the shortest path between transmitter and receiver. The LOS signal may pass through structures. The LOS path may be opaque in the visible spectrum. Multipath may contaminate decoded data signals with false data sequences and make detection of LOS data sequences difficult. Multipath may give rise to false sequences that have stronger signals than LOS signals because LOS signals may be attenuated by structures through which they pass.
It would be desirable, therefore, to provide efficient apparatus and methods for identifying wireless asset location.
It would also be desirable to provide apparatus and methods for accurately identifying wireless asset location.
It would be further desirable to provide apparatus and methods for identifying a location of a wireless asset broadcasting 802.11 signal structures.
It would be still further desirable to provide apparatus and methods for efficiently tagging passive assets for location identification.